Proteases are a rich source for the development of powerful therapeutics that can inactivate disease-causing proteins. We will create the first therapeutic protease targeted at infectious disease by neutralizing bacterial exotoxins. While most drugs are binders and act at a stoichiometric ratio, proteases are capable of catalytic turnover. Therapeutic proteases are currently used to treat thrombosis, sepsis, and coagulation, neuromuscular and digestion disorders. Significantly, all of these applications required identification of an existing protease in nature with the desired activity. Unfortunately, most potential therapeutic targets are not currently addressable because proteases with suitable specificities have not been identified. We will overcome this limitation by establishing two independent and synergistic methods that will enable the engineering of custom therapeutic proteases. In our first aim, we will develop a high-throughput method for assaying protease specificity. This technique will be invaluable for scanning natural proteases for activity that can be efficiently repurposed for therapeutic applications. Additionally, current methods are unable to reliably predict off-target cleavage within the human proteome. However, our unique approach will screen all possible octa-peptide substrates in a single experiment by combining our mRNA display technology, next-generation sequencing, LC-MS/MS, and computational analysis. The data from potentially millions of cleavage sequences will be compiled into a high-resolution specificity map. This map will facilitate the accurate prediction and subsequent elimination of protease cross-reactivity with human proteins. In our second aim, we will identify novel exotoxin-cleaving proteases from a library of over 1012 unique protein mutants by applying the mRNA display technology for the directed evolution of enzymes which we developed. This approach will enable the engineering of proteolytic specificity for multiple substrate residues simultaneously. As a result, we will be able to refine activity against potentially any protein target while minimizing off-target activity. In the first application of ou approach, we will design a protease to neutralize the Streptococcus pyogenes superantigen exotoxin SpeA, which has been linked to streptococcal toxic shock syndrome, necrotizing fasciitis, Kawasaki-like diseases, and many more diseases. Our overall goal is to establish methods to analyze and create novel protease specificities, which may open the path to a new class of protease therapeutics with broad applications from neutralizing the wide array of bacterial exotoxins in infectious disease to treating a variety of additional disorders that involv aberrant proteins.